In this study the distribution of insulin binding sites on the surface of rat adipocytes was studied using monomeric ferritin-insulin viewed by electronmicroscopy and analyzed by newly developed quantitative methods. This analysis shows that insulin binding sites are non randomly distributed on rat adipocyte plasma membrane. The quantitative characteristics of the receptor groups (e.g. distances between the sites) and the mode of action of cytochalasin B has been elucidated.